1. Field of the Invention
The present invention relates to a process for increasing the etch resistance of photoresists which are suitable for use in the production of microelectronic devices such as integrated circuits. More particularly, the invention provides a process for increasing the etch resistance of positive working 193 nm sensitive photoresists.
2. Description of the Related Art
The production of positive photoresists is well known in the art as exemplified by U.S. Pat. Nos. 3,666,473; 4,115,128 and 4,173,470. These contain aqueous alkali soluble polyvinyl phenol or phenol formaldehyde novolak resins together with light sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent and are applied as a thin film coating to a substrate suitable for the particular application desired. The resin component of photoresist formulations is soluble in an aqueous alkaline solution, but the photosensitizer is not. Upon imagewise exposure of the coated substrate to actinic radiation, the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in an alkaline developing solution, while the unexposed areas are substantially unaffected, thus producing a positive image on the substrate. The uncovered substrate is thereafter subjected to an etching process. Frequently, this involves a plasma etching against which the resist coating must be sufficiently stable. The photoresist coating protects the covered areas of the substrate from the etchant and thus the etchant is only able to etch the uncovered areas of the substrate. Thus, a pattern can be created on the substrate which corresponds to the pattern of the mask or template that was used to create selective exposure patterns on the coated substrate prior to development.
Photoresists are either positive working or negative working. In a negative working resist composition, the imagewise light struck areas harden and form the image areas of the resist after removal of the unexposed areas with a developer. In a positive working resist the exposed areas are the non-image areas. The light struck parts are rendered soluble in aqueous alkali developers. The ability to reproduce very small dimensions, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. As the integration degree of semiconductor devices becomes higher, finer photoresist film patterns are required. One way to increase circuit density on such a chip is by increasing the resolution capabilities of the resist. Positive photoresists have been found to be capable of much higher resolution and have almost universally replaced negative resists for this purpose.
The optimally obtainable microlithographic resolution is essentially determined by the radiation wavelengths used for the selective irradiation. However the resolution capacity that can be obtained with conventional deep UV microlithography has its limits. In order to be able to sufficiently resolve optically small structural elements, wavelengths shorter than deep UV radiation must be utilized. The use of UV radiation has been employed for many applications, particularly radiation with a wavelength of 193 nm. In particular, the radiation of argon fluoride excimer lasers, which has a wavelength of 193 nm is useful for this purpose. The deep UV photoresist materials that are used today, however, are not suitable for 193 nm exposure. Materials based on phenolic resins as a binding agent, particularly novolak resins or polyhydroxystyrene derivatives have too high an absorption at wavelengths below 200 nm and one cannot image through films of the necessary thickness. This high absorption at 193 nm radiation results in side walls of the developed resist structures which do not form the desired vertical profiles. Rather they have an oblique angle with the substrate which causes poor optical resolution characteristics at these short wavelengths. Polyhydroxystyrene based resists can be used in top surface imaging applications in which a very thin (.about.500 .ANG.) layer of resist is required to be transparent at the ArF wavelength. This invention involves the use of 193 nm resists in single layer processes.
Chemical amplification resist films have been developed, which have been found to have superior resolution. 193 nm photoresists are based on chemically amplified deprotection. With this mechanism, a molecule of photogenerated acid catalyzes the breaking of bonds in a protecting group of a polymer. During the deprotecting process, another molecule of the same acid is created as a byproduct, and continues the acid-catalytic deprotection cycle. The chemistry of a 193 nm photoresist is based on polymers such as, but not limited to, acrylates, cyclic olefins with alicyclic groups, and hybrids of the aforementioned polymers which lack aromatic rings, which contribute to opacity at 193 nm. It has thus been known to utilize photoresists based on methacrylate resins for the production of microstructures by means of 193 nm radiation.
However, chemically amplified resist films have not played a significant role in the fine pattern process using deep UV because they lack sufficient etch resistance, thermal stability, post exposure delay stability and processing latitude. While such photoresists are sufficiently transparent for 193 nm radiation, they do not have the etching stability customary for resists based on phenolic resins for plasma etching. A typical chemical amplification photoresist film comprises a polymer, a photoacid generator, and other optional additives. The polymer is required to be soluble in the chosen developer solution, and have high thermal stability and low absorbance to the 193 nm exposure wavelength in addition to having excellent etch resistance. Since resists containing aromatic compounds show high absorbance to ArF (193 nm) while non-aromatic matrix resins have a poor etch resistance, these contrasting weak points are factors retarding the development of excellent photoresist films for ArF lithography. It would be desirable to overcome the above mentioned problems and to provide a photoresist film superior in etch resistance, as well as transmittance to deep UV.
There have been several attempts to solve this problem. One attempt to improve the etching stability of photoresists based on meth(acrylate) introduced cycloaliphatic groups into the meth(acrylate) polymers. This leads to an improvement in etching stability, but not to the desired extent. Another proposal aims at producing sufficient etching stability only after irradiation in the resist coating. It has been proposed to treat the substrate with the finished, developed, image-structured photoresist coating with specific alkyl compounds of magnesium or aluminum, in order to introduce the given metals in the resist material as etching barriers (See U.S. Pat. No. 4,690,838). The use of metal-containing reagents, however, is generally not desired in microlithography process, due to the danger associated with contamination of the substrate with metal ions.
It has now been found according to the present invention, that by subjecting a developed photoresist to electron beam irradiation, a resist image is produced which is still sufficiently transparent for radiation with a wavelength of approximately 193 nm and which is now sufficiently stable to permit plasma etching. In this way, it is possible to produce a photoresist which can be exposed at approximately 193 nm wavelength, which also has an etching rate that is comparable to conventional resists based on phenolic resin, without needing to treat the resist coating with metal compounds in order to increase the etching stability. The present invention therefore provides a process for increasing the etch stability of photoresist compositions which are transparent at a wavelength of approximately 193 nm.